1. Technical Field
The present invention relates to an ink for functional layer formation, a method for manufacturing a light emitting element using the ink for functional layer formation, a light emitting device, and an electronic apparatus.
2. Related Art
In recent years, a method using a liquid-phase process has been proposed as a method for forming a functional layer, such as a colored layer for a color filter (filter layer), a light emitting layer for an organic electroluminescence element, and a semiconductor layer for an organic thin film transistor. Among the liquid-phase processes, ink jet methods (also referred to as liquid droplet discharge methods) has garnered attention as methods able to precisely coat a predetermined position with a predetermined amount of ink as a liquid droplet from an ink jet head.
Meanwhile, in the ink (solution) used in the ink jet method, clogging of the nozzles occurs when the solid content precipitates on the nozzles of the ink jet heat due to drying, and discharge defects in which the landing position of the liquid droplets varies or the discharge amount of the liquid droplets discharged from the nozzles varies occur. Accordingly, there is demand for an ink (solution) in which discharge defects do not easily occur. Since the functional layer (functional film) is formed by coating and drying the ink (solution), there is demand for an ink that is able to coat a predetermined region on a coating target, such as a substrate, without irregularities.
Japanese Patent No. 4616596 discloses an ink composition for amorphous film formation that includes a first solvent with an organic material solubility of 0.5 wt % or greater, and a second solvent formed of an alcohol compound with boiling point of 120° C. or higher at an organic material solubility of 0.1 wt % or less, and the boiling point of the first solvent is higher than the boiling point of the second solvent. According to Japanese Patent No. 4616596, it is disclosed that ink composition for amorphous film formation is suitable to an ink jet method.
For example, Japanese Patent No. 4707658 discloses a coating solution that includes a mixed solvent and functional material, in which the mixed solvent has two or more aromatic rings and includes a first solvent configured by a compound having a symmetrical structure and another solvent. By using the coating solution, clogging of the nozzles in the ink jet method does not easily occur, and it is possible for stable discharge to be realized.
For example, Japanese Patent No. 5019454 discloses an organic semiconductor solution that includes at least one type of organic solvent A that is a good solvent for an organic semiconductor and at least one type of organic solvent B that is a poor solvent for of an organic semiconductor, in which the boiling point of the organic solvent A is higher than the boiling point of the organic solvent B. According to the solution, the organic semiconductor material does not easily aggregate and precipitate during drying, homogeneous film formation is possible, and it is possible for an organic semiconductor having stable electrical characteristics to be formed.
However, in the ink jet method, when ink is filled in a film formation region surrounded by a partition wall, and dried and solidified in a state in which ink seeps up the side wall of the partition wall, a phenomenon arises in which the film thickness of the functional layer after solidifying becomes thicker at the partition wall side compared to the center portion. According to the movement state of solute in the ink during the drying process, the film thickness in the center portion becomes thick compared to the other parts. That is, a problem arises in which the film thickness in the film formation region fluctuates and it is difficult to obtain a functional layer with a flat cross-sectional shape.
For example, in cases in which a light emitting layer is included in the functional layer, when the film thickness fluctuates, brightness irregularities, and color irregularities in light emission color occur.